1. Field of the Invention
The present invention relates generally to a plasma source for deposition of thin films and chemical modification of surfaces. More particularly, the present invention relates to a linear plasma source for plasma enhanced chemical vapor deposition (CVD).
2. Discussion of the Background
All United States Patents and Patent Applications referred to herein are hereby incorporated by reference in their entireties. In the case of conflict, the present specification, including definitions, will control.
The deposition of thin films can be accomplished by many techniques, the most common including chemical deposition, physical deposition and mixtures of the two. For chemical deposition, well-known techniques are plating, chemical solution deposition (CSD) and chemical vapor deposition (CVD). Plating and CSD generally utilize liquid chemical precursors while CVD generally utilizes gaseous chemical precursors. These techniques can be done at atmospheric pressure or under vacuum conditions. For physical deposition, well-known techniques are thermal evaporation, sputtering, pulsed laser deposition and cathodic arc deposition. These physical deposition techniques generally utilize vacuum conditions in order to deposit the desired thin film materials. With respect to chemical deposition, the most common technique is CVD, whereas for physical deposition, the most common technique is sputtering.
CVD generally requires that an energy source be included in order to create conditions such that a precursor gas will adhere, or stick, to a substrate surface. Otherwise, adhesion to a surface will not occur. For example, in a pyrolytic CVD process whereby a thin film coating is desired on a flat glass substrate, it is typical for the glass substrate to be heated. The heated glass substrate acts as the CVD energy source and when the precursor gas contacts the heated glass substrate, the precursor gas adheres to the hot glass surface. The heated surface also provides the energy needed to cause the precursor gas to chemically react to form the final thin film coating composition.
A plasma is also able to act as the energy source for CVD type processes, known as plasma enhanced CVD, or PECVD. A plasma is composed of a partially ionized gas and free electrons, and each component has the ability to move somewhat independently. This independent movement makes the plasma electrically conductive, such that it can respond to electromagnetic fields. This electrical conductivity provides PECVD processes with a number of advantages over other known chemical and physical deposition techniques.
In a PECVD process, the depositing material is typically derived from a precursor gas. Examples of such precursor gases are well-known to those of skill in the art. For example, if an Si-based thin film is to be deposited, a common precursor gas is silane, SiH4. When SiH4 is subject to a source of plasma, the plasma can act to raise the energy level of the silane molecule to the point where it will react with a surface and attach as a solid layer. More specifically, the SiH4 becomes ionized, with its electrons moving to a higher energy level. This is accompanied by subsequent stripping off of the hydrogen atoms. The ionized molecules have open reactant sites available and, if in the presence of a reactant gas such as oxygen, can readily form a thin film of SiO2. If the ionized molecules are not in the presence of a reactant gas, a thin film of silicon can be formed. The precursor gas chemistry exists for a plethora of elements, and thus, there is a large availability of elements and materials that can be deposited by PECVD. Without limitation, the types of thin films that can be deposited by PECVD are transparent conductive oxide thin film coatings, solar control and optical thin film coatings and semiconductor thin film coatings. Other types of thin film coatings that are able to be deposited by PECVD will be recognized and appreciated by those of ordinary skill in the art.
Thus, creating a plasma in proximity to a surface is a common industrial practice, particularly in the coating industry. Many devices have been developed to create and shape plasmas. Most known devices create a cylindrically shaped plasma plume, which have numerous practical applications for coatings and surface treatment. However, linear plasmas potentially have more practical applications. Linear plasmas can be made to work over large substrate surface areas, which is useful for large area glass coating, web coating and multipart batch coating.
To date, most known PECVD apparatuses are for small scale (i.e. <1 m2) depositions since most plasma sources are very short and can only coat small areas. Thus, PECVD applied to large area coating has been difficult to implement. However, there have been PECVD apparatuses designed for coating large area surfaces. These include, without limitation, magnetron sources, anode layer ion sources and Madocks sources.
However, there are drawbacks associated with using the aforementioned PECVD apparatuses for coating large area surfaces. Magnetron sources, for example, tend to quite bulky, typically 150 mm wide by 300 mm deep, and require magnets. Also, when used for PECVD, the surface of a magnetron source tends to become coated with the material being deposited, and thus, the magnetron becomes insulated, which can lead to arcing and other complications. Furthermore, the sputtered material contaminates the material being deposited. Anode ion layer sources, for example, suffer from similar drawbacks as magnetron sources in that they tend to be bulky, require magnets, as well as become coated. Furthermore, anode ion layer sources tend to deposit PECVD materials at a low rate, 0.1 μm/second. Madocks sources, for example, suffer from the drawbacks of being bulky and requiring magnets, as well as low coating efficiencies, about 15%. Moreover, all three of the aforementioned sources rely on closed circuit electron drift (e.g., the Hall effect) to create a uniform plasma.
It is possible to create a uniform plasma without the reliance upon closed circuit electron drift, or the Hall effect. A common approach to doing this is to have two electron emitting surfaces aligned substantially parallel with respect to each other, wherein the electron emitting surfaces are connected to each other in a bipolar and out of phase manner via an AC power source. When a voltage difference is applied to both electron emitting surfaces, a plasma can be created. The polarities between the two electron emitting surfaces are switched from positive to negative at some predetermined frequency and the plasma becomes spread out and uniform.
Plasma sources based on parallel electron emitting surfaces have been developed. One such source is a hollow cathode source, such as that described in U.S. Pat. No. 6,444,945. More specifically, the plasma source described in U.S. Pat. No. 6,444,945 includes a structure made up of two hollow cathode shapes connected to a bipolar AC power supply, as shown in FIG. 1. The plasma source includes first and second hollow cathode structures 1 and 2. The two hollow cathode structures 1 and 2 are electrically connected by wires 6 to an AC power source 5 which generates an AC current to drive the formation of plasma 3. While one of the hollow cathode structures is subjected to a negative voltage, the other hollow cathode structure is subjected to a positive voltage, creating a voltage difference between the hollow cathode structures and causing current to flow between the structures, thereby completing the electric circuit. Optionally, magnets 4 may be disposed in proximity to the openings of each hollow cathode to enhance the plasma current between hollow cathode structures 1 and 2. However, U.S. Pat. No. 6,444,945 does not address the use of the disclosed hollow cathodes for any PECVD process or for large area surface coating.
Thus, there remains a need in the large area coating art for a plasma source, or a PECVD source, that can create a uniform and stable plasma of considerable length, i.e., greater than 0.5 meters in length. There also remains a need in the art for a PECVD source that is compact and can deposit a coating with high coating efficiencies. There further remains a need in the art for a PECVD source and process that consumes less energy during operation such that overall operating costs are reduced.